ZOOLOGY 



SECT 



FIG. 1207. Brain of Kangaroo (Macropits 

 major). (After Owen.) 



smooth, or nearly smooth, surfaces. In the higher types the 

 relative development of the hemispheres is immense, and their 

 backward extension causes them to cover over all the rest of the 



brain, while the cortex is thrown 

 into numerous complicated con- 

 volutions separated by deep 

 sulci (Fig. 1208). This develop- 

 ment of the cerebral hemispheres 

 reaches its maximum in Man. 



The organs of special sense 

 have the same general structure 

 and arrangement as in the 

 Sauropsida. Jacdbsons organs, 

 which in the Sauropsida consti- 

 tute such important accessory 

 parts to the olfactory apparatus, 

 are well developed only in the 

 lower groups of Mammals. The 

 olfactory mucous membrane is of 

 great extent, owing to the de- 

 velopment of the convoluted 

 ethmo-turbinal bones over which 

 it extends. In the toothed 

 Cetacea alone among Mammals 

 do the nasal chambers lose their sensory functions the olfactory 

 nerves being vestigial or absent. The organs of taste are taste- 

 buds in the mucous membrane covering certain of the papillae 

 on the surface of the tongue. 



In essential structure the eye of the Mammal resembles that of 

 the Vertebrates in 

 general (see p. 109). 

 The sclerotic is com- 

 posed of condensed 

 fibrous tissue. The 

 pecten of the eye of 

 Birds and Reptiles is 

 absent. In most Mam- 

 mals there are three 

 movable eyelids, two, 

 upper and lower, 

 opaque and usually 

 covered with hair, and 

 one anterior, translu- 

 cent, and hair-less 

 the nictitating membrane. The secretions of a lacrymal, a Harderian 

 and a series of Meibomian glands moisten and lubricate the surface 

 of the eye-ball and its lids. In Moles, and certain other burrowing 



FIG. 1208. Dorsal view of brain of Gray's Whale 

 (Coyia grayi). (After Haswcll.) 



